Spectral imaging

ABSTRACT

An imaging system includes radiation source ( 106 ) that emits radiation that traverses an examination region and a portion of a subject therein and a detector array ( 114 ) that detects radiation that traverses the examination region and the portion of the subject therein and generates a signal indicative thereof. A volume scan parameter recommender ( 120 ) recommends at least one spectral scan parameter value for a volume scan of the portion of the subject based on a spectral decomposition of first and second 2D projections acquired by the radiation source and detector array. The first and second 2D projections have different spectral characteristics. A console ( 122 ) employs the recommended at least one spectral scan parameter value to perform the volume scan of the portion of the subject.

The following generally relates to spectral imaging and moreparticularly to determining a suitable tube voltage for a spectralvolume scan based on two or more two-dimensional (2D) projection scans(scanograms) performed at two or more different kVps from approximatelythe same acquisition view, and is described with particular applicationto computed tomography (CT).

A conventional CT scanner generally includes an x-ray tube mounted on arotatable gantry, opposite a detector array, across an examinationregion. The rotatable gantry, and hence the x-ray tube, rotates aroundthe examination region. The x-ray tube emits radiation that traversesthe examination region and a portion of a subject therein and isdetected by the detector array. The detector array generates and outputsa signal indicative of the scanned portion of a subject. The signal isreconstructed to generate three dimensional volumetric image dataindicative of the scanned portion of a subject.

The volumetric image data includes voxels represented in terms of grayscale values corresponding to relative radiodensity. The gray scalevalues reflect the attenuation characteristics of the scanned subject,and generally show anatomical structures within the scanned subject.Since the absorption of a photon by a material is dependent on theenergy of the photon traversing the material, the detected radiationalso includes spectral information. However, conventional CT image datadoes not reflect the spectral information as the signal is proportionalto an energy fluence integrated over the energy spectrum.

A spectral CT scanner captures the spectral information, which isindicative of the elemental or material composition (e.g., atomicnumber) of the material of the scanned portion of the subject. SpectralCT scanner configurations include two or more x-ray tubes angularlyoffset from each other and configured to emit radiation having differentpeak emission voltages, a single x-ray tube configured to switch betweenat least two different kVps (e.g., 80 kVp and 140 kVp), a single broadspectrum x-ray tube and an energy-resolving detectors, and/or acombination thereof.

Volume scans typically are planned (i.e., scan position and scan length)using a 2D projection image from a 2D projection scan at a fixed kVp.However, the 2D projection image generally does not provide sufficientinformation to optimally select scan parameters such as kVps for aspectral volume scan, and image quality depends on the kVp. For example,good spectral performance can be achieved with a lower value (e.g. 80kVp) for the lower kVp. However, with a larger subject, fewer 80 kVpphotons pass through the subject, and the volumetric image data may benoisy with degraded image quality.

Aspects described herein address the above-referenced problems andothers.

In one aspect, an imaging system includes radiation source that emitsradiation that traverses an examination region and a portion of asubject therein and a detector array that detects radiation thattraverses the examination region and the portion of the subject thereinand generates a signal indicative thereof. A volume scan parameterrecommender recommends at least one spectral scan parameter value for avolume scan of the portion of the subject based on a spectraldecomposition of first and second 2D projections acquired by theradiation source and detector array. The first and second 2D projectionshave different spectral characteristics. A console employs therecommended at least one spectral scan parameter value to perform thevolume scan of the portion of the subject.

In another aspect, a method includes receiving first 2D projectionhaving a first spectral characteristic and receiving second 2Dprojection having a second spectral characteristic, wherein the firstand second spectral characteristics are different. The method furtherincludes spectrally decomposing the first and second data into at leasttwo different components. The method further includes determining aphysical characteristic of the scanned subject based on the decomposeddata. The method further includes determining at least one spectral scanparameter value for a volume scan of the subject based on the determinedphysical characteristic. The method further includes performing thevolume scan of the subject using the at least one spectral scanparameter value.

In another aspect, a computer readable storage medium is encoded withcomputer readable instructions. The computer readable instructions, whenexecuted by a processer, cause the processor to acquire several 2Dprojections at least at two different emission spectrums from at leastone acquisition angle and determine at least one spectral scan parametersettings for a volume scan based on the acquired several 2D projections.

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating the preferred embodiments and arenot to be construed as limiting the invention.

FIG. 1 schematically illustrates an example imaging system in connectionwith a volume scan parameter recommender.

FIG. 2 schematically illustrates an example of the volume scan parameterrecommender.

FIG. 3 schematically illustrates a multi-layer scintillator/photosensorbased energy-resolving detector.

FIG. 4 schematically illustrates another multi-layerscintillator/photosensor based energy-resolving detector.

FIG. 5 schematically illustrates direct conversion photon countingdetector output processing electronics.

FIG. 6 illustrates an example method for determining a kVp for aspectral volume scan.

FIG. 1 schematically illustrates an imaging system 100 such as acomputed tomography (CT) scanner 100. The imaging system 100 includes astationary gantry 102 and a rotating gantry 104. The rotating gantry 104is rotatably supported by the stationary gantry 102. The rotating gantry104 is configured to rotate around an examination region 108 about alongitudinal or z-axis.

At least one radiation source 106, such as an x-ray tube, is supportedby the rotating gantry 104 and rotates therewith. The rotating gantry104 rotates to move the radiation source 106 to a predetermined staticangular location, if not already at the location, for example, for a 2Dprojection scan. The rotating gantry 104 also rotates to rotate theradiation source 106 around the examination region 108 for volume scan.

A radiation source voltage determiner 112 is configured to switch thepeak emission voltage between two or more voltages (e.g., 80 kVp, 100kVp, 140 kVp, etc.) for 2D projection and/or volume scans. By way ofnon-limiting example, for a 2D projection scan, one kVp can be used foran anterior-posterior (AP) projection scan (and/or a lateral scan) inone direction along the z-axis and another kVp can be used for an APscan (and/or a lateral scan) in the same or an opposing direction alongthe z-axis.

Alternatively, two different 2D projections at different kVps and atapproximately a same acquisition angle can be acquired through fast kVswitching and a backward jump of the focal spot. This may facilitatereducing mismatch artifact due to gantry motion. The lower kVp value,generally, is high enough to mitigate photon starvation caused by theabsorption of all or substantially all of the photons. The radiationsource voltage determiner 112 can also be used to drive the source 106with a single fixed kVp.

A one or two dimensional radiation sensitive detector array 114 subtendsan angular arc opposite the examination region 108 relative to theradiation source 106. The detector array 114 detects radiation thattraverses the examination region 108 and generates an output signalindicative thereof. The illustrated detector array 114 includes aphotosensor array 116 with photosensors such as photodiodes or the likeand a scintillator array 118, which is optically coupled to thephotosensor array 116 on a light sensitive side of the photosensor array116.

The detector array 114 is arranged in the scanner 100 so that thescintillator array 118 receives radiation emitted by the source 106 thatilluminates the detector array 114. Alternatively, the detector array114 may include an energy-resolving detector array such as a multi-layerscintillator/photosensor detector array (FIGS. 3 and 4 below), a directconversion photon counting detector array with corresponding electronics(FIG. 5), and/or other energy-resolving detector. With a multi-layerscintillator/photosensor detector array, a single or multiple kVps canbe used for a multi-energy acquisition. In general, m kVps can be usedwith n different detector layers for m×n energy acquisitions andenergy-dependent detector output signals. With a direct conversionphoton counting detector, a single kVp is used to acquire oneprojection, and spectral information is obtained by pulse heightanalysis of the electrical output signal of the direct conversiondetector.

A volume scan parameter recommender 120 recommends one or more spectralvolume scan protocol parameters such as a tube voltage (kVp), tubecurrent (mAs), energy threshold (for photon counting detectors), and/orother spectral scan protocol parameters. The illustrated recommender 120spectrally decomposes the 2D projections and recommends the parameterbased on the spectral decomposition. Generally, for spectraldecomposition, data for two different kVps acquired at approximately thesame acquisition angle is decomposed into at least two differentcomponents, for example, bone and soft tissue. An example spectraldecomposition is discussed in Alvarez, R. E. & Macovski, A.“Energy-selective Recon in X-ray Computerized Tomography”, Phys MedBiol, 1976, 21, 733-44.

Briefly turning to FIG. 2, a non-limiting example of the recommender 120is schematically illustrated. A characteristic determiner 202 determinesa characteristic of the scanned subject from the decomposed data. Forexample, based on the decomposition, the recommender 120 can extrapolatean attenuation of the object, even at lower kVp settings. From thisdata, the recommender 120 can determine whether a volume scan at aparticular kVp (e.g., 80 kVp) would suffer from photon starvation. Thisallows the recommender 120 to recommend a proper kVp value for thevolume scan that trades off the risk of photon starvation and a goodspectral performance.

In one non-limiting instance, the recommender 120 makes thisdetermination based on a characteristic to kVp map 204, which mapsattenuation values (and/or other characteristics) to kVp values andwhich is based on one or more predetermined tradeoffs between risk ofphoton starvation and good spectral performance and/or other criteria.For example, the characteristic determiner 202 locates the determinedcharacteristic(s) in the characteristic to kVp map 204 and retrieves akVp value from the map 204. The map 204 can be generated during acalibration with an appropriate phantom and stored as a look up table(LUT) and/or otherwise. The retrieved kVp value can be automaticallyloaded in the system 100, displayed for operator review, used to notifythe operator that a kVp setting may result in detector overflow orhigher patient dose, etc.

By way of non-limiting example, a first 2D projection scan performed at100 kVp and a second 2D projection scan performed at 140 kVp may bespectrally decomposed to provide data that indicates that the lower kVpsetting of a dual-energy scan can be set lower than 100 kVp such as to80 kVp. Alternatively, the characteristic(s) may indicate that the lowerkVp setting should be 100 kVp or greater. Other approaches are alsocontemplated herein. For example, the recommender 120 may alternativelycompute one or more kVp values based on a set of rules, an algorithm,and/or other information.

Generally, the recommender 120 allows for determining and/or optimizingone or more kVps for a spectral volume scan. The volume scan parameterrecommender 120 can be implemented via at least one processor executingcomputer executable instructions embedded or encoded on computerreadable storage medium such as physical memory or other non-transitorymedium. Additionally or alternatively, the volume scan parameterrecommender 120 can be implemented by the at least one processorexecuting computer executable instructions carried by a carrier wave,signal, or other transitory medium.

Returning to FIG. 1, a general-purpose computer serves as an operatorconsole 122 and includes a human readable output device such as amonitor or display and an input device such as a keyboard and mouse.Software resident on the console 122 allows the operator to interactwith the scanner 100 via a graphical user interface (GUI) or otherwise.Such interaction includes selecting two or more 2D projection scans(e.g., a multi-2D projection scan mode), setting and/or modifying scanparameters (e.g., kVp, mAs, etc.) for selected 2D projection scans,selecting a spectral volume scan protocol, setting and/or modifyingprotocol parameters (e.g., kVps, mAs, etc.) for a selected spectralvolume scan.

The interaction may also include visual presentation via the GUI of therecommended parameters (e.g., kVps) and allows for confirmation,modification and/or rejection thereof by the operator of the imagingsystem 100 using a mouse, touch screen, keyboard, keypad, etc.Alternatively, the recommended parameters may be automatically loaded inthe system 100. Alternatively, where the recommended parameters aredifferent from corresponding user selected or default parameters, theconsole 122 may notify the operator, via visual and/or audibleindicator, with a message or warning.

A reconstructor 124 reconstructs the detector array output signal andgenerates volumetric image data. The reconstructor 124 employs one ormore of a conventional, a spectral, an iterative, and/or otherreconstruction algorithm. The volumetric image data may include datacorresponding to a specific spectral component and/or composite datasimilar to convention CT image data. The reconstructor 124 can alsogenerate 2D projection images corresponding to a specific spectralcomponent. A subject support 126 such as a couch supports a subject inthe examination region 108. The subject support 126 is movable incoordination with scanning so as to guide the subject with respect tothe examination region 108 for performing a 2D projection and/or volumescans.

Variations are discussed.

As briefly noted above, the detector array 114 may alternatively includea multi-layer scintillator/photosensor detector array and/or directconversion photon counting detector array. FIGS. 3 and 4 schematicallyshow non-limiting examples of two-layer scintillator/photosensordetector arrays 300 and 400, and FIG. 5 show processing electronics forprocessing the output of a direct conversion photon counting detectorarray.

In FIG. 3, first and second layers 302 and 304 of scintillation materialare coupled such that the first layer 302 is on a side of the detectorreceiving incoming radiation. Photosensors 310 and 312 are coupled to asecond opposing side of the detector 300. Energy absorption, generally,is dependent on thickness. As such, the majority of lower energy photonsare absorbed in the first layer 302 and the majority of higher energyphotons are absorbed in the second layer 304. The scintillator layers302 and 304 respectively have emission spectra that match the spectralsensitivities of the photosensors 310 and 312.

As such, substantially only the light emitted by the first layer 302 isabsorbed by the photosensor 310, and substantially only the lightemitted by the second layer 304 is absorbed by the photosensor 312. Thephotosensors 310 and 311 output signals indicative of radiation fromdifferent energy bands, which correspond to the first and second layers302 and 304. In FIG. 4, the photosensors 310 and 312 are coupled to aside of the scintillator layers 302 and 304, which is perpendicular tothe direction of incoming radiation.

Turning to FIG. 5, the output of the detector array 114 is processed bya pre-amplifier 502, which amplifies the electrical signal output by thedetector array 114. A pulse shaper 504 processes the amplifiedelectrical signal and generates a pulse such as voltage or other pulseindicative of the energy of the detected photon. Energy discriminator506 energy discriminates the pulse. In the illustrated example, theenergy discriminator 506 includes a comparator 508, including at leasttwo sub-comparators, which compares the amplitude of the pulse with twoor more different energy thresholds that correspond to differentenergies of interest. The comparator 508 produces an output signalindicative of the energy of the photon based on the comparison.

A counter 510 increments a count value for each threshold based on theoutput of the energy discriminator 506. For instance, when the output ofthe comparator 508 for a particular threshold indicates that theamplitude of the pulse exceeds the corresponding threshold, the countvalue for that threshold is incremented. Energy binner 512 assigns thecounted pulses to two or more energy bins based on the counts. Eachenergy bin encompasses an energy range or window. For example, a bin maybe defined for the energy range between two thresholds, where a photonresulting in a count for the lower threshold but not for higherthreshold would be assigned to that bin.

The volume scan parameter recommender 120 processes the binned data andthe recommender 120 recommends spectral scan parameters such as kVp,mAs, one or more energy thresholds, and/or other parameter(s).

Other variations may include more scintillation layers, and theindividual scintillation layers may have equal thickness and/ordifferent thickness. With dual kVps and a dual-layer detector, therecommender 120 recommends settings for four different energyacquisitions. In general, with M kVps and N layers, the recommender 120recommends settings for M×N different energy acquisitions. With respectto a photon counting detector, with N energy bins, the recommender 120recommends settings for N different energy acquisitions.

In another variation, the 2D projections can additionally oralternatively be used to determine a filter that improves the spectralperformance of a dual layer system. In the case where filter improvesthe performance only for small patients and degrades the performance forthick patients, the recommender 120 recommendations whether the filtershould be used.

In another variation, the imaging system 100 includes more than onesource 106. For example, the imaging system 100 may include two sources106 angularly offset from one another by about ninety degrees along atransverse direction, which is perpendicular to the z-axis, threesources 106 angularly offset from one another by about sixty degreesalong the transverse direction, etc. Each of these sources can beoperated as described herein to switch between kVps and/or pair with anenergy-resolving detector array.

In another variation, the recommender 120 uses the decomposition of the2D projections to determine a tube current (mAs) for the source 106.

In another variation, the recommender 120, for kVp switching, canrecommend a kVp cycling parameter. For example, the recommender 120might recommend how many projections are acquired at each kVp setting.As such, a ratio of data acquired with the two kVp settings can beadjusted in order to satisfy predetermined output criteria. Forinstance, the criteria may indicate that the data for both kVp settingshave the same image quality. Thus, for a thicker patient, therecommender 120 may recommend more projections at the lower kVp settingrelative to the higher kVp setting or alternatively longer acquisitionperiods for the lower kVp settings relative to the higher kVp settingsto compensate for poorer statistics. FIG. 6 illustrates an examplemethod.

It is to be appreciated that the ordering of the acts in the methodsdescribed herein is not limiting. As such, other orderings arecontemplated herein. In addition, one or more acts may be omitted and/orone or more additional acts may be included.

At 602, a first 2D projection is acquired at an acquisition angle usinga first kVp.

At 604, a second 2D projection is acquired at the acquisition angleusing a second kVp, which is different from the first kVp.

At 606, the first and second 2D projections are spectrally decomposedinto at least two components.

At 608, at least one characteristic (e.g., thickness, amount of boneand/or soft tissue, etc.) of the scanned subject is determined from thespectrally decomposed data.

At 610, at least one kVp value for a spectral volume scan of the subjectis determined based at least on the determined physical characteristic.

At 612, the determined kVp, along with at least one other determined oruser defined kVp, is used to perform the spectral volume scan.

As described herein, the first and second 2D projections mayalternatively be obtained with a single kVp and either a multi-layerscintillator/photosensor detector array, direct conversion photoncounting detector array, and/or other energy-resolving detector array.

At least a portion of the above may be implemented by way of computerreadable instructions, encoded or embedded on computer readable storagemedium, which, when executed by a computer processor(s), cause theprocessor(s) to carry out the described acts. Additionally oralternatively, at least one of the computer readable instructions iscarried by a signal, carrier wave or other transitory medium.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be constructed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. An imaging system, comprising: a radiation source that emitsradiation that traverses an examination region and a portion of asubject therein; a detector array that detects radiation that traversesthe examination region and the portion of the subject therein andgenerates a signal indicative thereof, a volume scan parameterrecommender that recommends at least one spectral scan parameter valuefor a volume scan of the portion of the subject based on a spectraldecomposition of first and second 2D projections acquired by theradiation source and detector array, wherein the first and second 2Dprojections have different spectral characteristics; and a console thatemploys the recommended at least one spectral scan parameter value toperform the volume scan of the portion of the subject.
 2. The imagingsystem of claim 1, wherein the spectral scan parameter is a kVp value,and the radiation source is configured to switch between at least twodifferent kVp values, a first kVp value which is used to acquire thefirst 2D projection and a second different kVp value which is used toacquire the second 2D projection, wherein the first kVp value is lessthan the second kVp value.
 3. The imaging system of claim 2, wherein theat least one recommended kVp value is equal to the first kVp.
 4. Theimaging system of claim 2, wherein the at least one recommended kVpvalue is less than the first kVp value.
 5. The imaging system of claim2, wherein the console employs both the at least one kVp value and thesecond kVp value to perform a multi-spectral volume scan.
 6. The imagingsystem of claim 2, wherein the detector array is an energy resolvingdetector array having at least first and second photosensors with firstand second different spectral sensitivities, and the first photosensorgenerates the first 2D projection and a third 2D projection having afirst and a third spectral characteristic, and the second photosensorgenerates the second 2D projection and a fourth 2D projection having asecond and a fourth spectral characteristic, and the volume scanparameter recommender recommends the at least one kVp value based on aspectral decomposition of the first, second, third and fourth 2Dprojections.
 7. The imaging system of claim 1, wherein the detectorarray is an energy resolving detector array having at least first andsecond photosensor with first and second different spectralsensitivities, and the first photosensor generates the first 2Dprojection and the second photosensor generates the second 2Dprojection.
 8. The imaging system of claim 1, wherein the detector arrayis a photon counting detector, the first 2D projection corresponds to afirst energy bin of the photon counting detector, the second 2Dprojection corresponds to a second energy bin of the photon countingdetector, and the at least one spectral scan parameter includes one ormore of a kVp value, a mAs value or an energy threshold value.
 9. Theimaging system of claim 1, wherein the console visually presents therecommended at least one spectral scan parameter value in a graphicaluser interface.
 10. The imaging system of claim 9, wherein the consolereceives an input signal indicative of a confirmation of the at leastone spectral scan parameter value.
 11. The imaging system of claim 9,wherein the console receives an input signal indicative of a change tothe at least one spectral scan parameter value and loads the confirmedat least one spectral scan parameter value for the volume scan.
 12. Theimaging system of claim 1, wherein the console visually presents awarning indicating that the at least one spectral scan parameter valueis not the same as a user selected spectral scan parameter for thevolume scan.
 13. A method, comprising: receiving first 2D projectionhaving a first spectral characteristic; receiving second 2D projectionhaving a second spectral characteristic, wherein the first and secondspectral characteristics are different; spectrally decomposing the firstand second data into at least two different components; determining aphysical characteristic of the scanned subject based on the decomposeddata; determining at least one spectral scan parameter value for avolume scan of the subject based on the determined physicalcharacteristic; and performing the volume scan of the subject using theat least one spectral scan parameter value.
 14. The method of claim 13,wherein the first 2D projection corresponds to a first 2D projectionscan of the subject performed at a first kVp value at a predeterminedacquisition angle; and the second 2D projection corresponds to a second2D projection scan of the subject performed at a second different kVpvalue at the predetermined acquisition angle, wherein the second kVpvalue is higher than the first kVp value.
 15. The method of claim 14,wherein the at least one kVp value is equal to the first kVp.
 16. Themethod of claim 14, wherein the at least one kVp value is less than thefirst kVp value.
 17. The method of claim 14, further comprising:perforating the volume scan of the subject using the at least one kVpvalue and the second kVp value.
 18. The method of claim 13, wherein thefirst 2D projection corresponds to an output of a first photosensor of adetector pixel having a first spectral sensitivity and the second 2Dprojection corresponds to an output of a second photosensor of thedetector pixel having a second first spectral sensitivity, which isdifferent from the first spectral sensitivity.
 19. The method of claim13, further comprising: visually presenting the at least one spectralscan parameter value; receiving an input signal indicative of a userconfirmation of the at least one kVp value; and performing the volumescan of the subject using the user confirmed at least one spectral scanparameter value.
 20. The method of claim 13, further comprising:determining the at least one spectral scan parameter value is not thesame as a user selected spectral scan parameter for the scan; andvisually presenting a warning indicating that the at least one spectralscan parameter value is not the same as a user selected spectral scanparameter for the scan.
 21. The method of claim 13, further comprising:automatically loading the at least one spectral scan parameter value inan imaging protocol for the volume scan; and performing the volume scanof the subject using the automatically loaded at least one spectral scanparameter value.
 22. The method of claim 13, further comprising:determining at least one mAs value for the volume scan of the subjectbased on the determined physical characteristic; and performing thevolume scan of the subject using the at least one mAs value.
 23. Themethod of claim 13, further comprising: determining a kVp switchingpattern that compensate for poorer photon statistics of the at least onekVp relative to a second higher kVp by acquiring more projections withthe at least one kVp setting; and performing the volume scan of thesubject based on the switching pattern.
 24. The method of claim 13,wherein the first 2D projection corresponds to a first energy bin ofphoton counting detector electronics and the second 2D projectioncorresponds to a second energy bin of the photon counting detectorelectronics, and the at least one spectral scan parameter includes oneor more of a kVp value, a mAs value or an energy threshold value.
 25. Acomputer readable storage medium encoded with computer readableinstructions, which, when executed by a processer, causes the processorto: acquire several 2D projections at least at two different emissionspectrums from at least one acquisition angle; and determine at leastone spectral scan parameter settings for a volume scan based on theacquired several 2D projections.
 26. A method, comprising: recommendingat least one kVp value based on spectral decomposition data, wherein therecommended at least one kVp value balances a photon count and aspectral resolution of an image; and visually presenting the recommendedat least one kVp value via a display.